With a current demand for three-dimensional stacking or high-density packaging of semiconductor devices, it is important nowadays to provide thin semiconductor wafers. One method to provide a thin semiconductor wafer is to lap (grind) the back surface of a semiconductor wafer with a grind stone. While the method is pervasive for it offers good productivity, a drawback of the method is that it may produce micro cracks in the back surface of the semiconductor wafer when the semiconductor wafer is ground, with the result that the bending strength of the semiconductor wafer may be reduced. In order to prevent chipping or cracking that may develop in the semiconductor wafer by an applied external force of grinding, the method requires the semiconductor wafer to be supported (anchored) on a reinforcing member when grinding the back surface of the semiconductor wafer.
The mechanical strength of the semiconductor wafer is weak, and the method requires a “stress-free” technique in which the semiconductor wafer is removed (detached) from the reinforcing member without exerting stress. This is particularly important when a thin semiconductor wafer is provided, in which case the mechanical strength of the semiconductor wafer is even weaker.
In order to grind the semiconductor wafer by anchoring it on a reinforcing member, it is required (1) to anchor the semiconductor wafer on the reinforcing member with such an adhesion force that can withstand back grinding, and (2) to detach the semiconductor wafer from the reinforcing member without exerting stress on the semiconductor wafer that was ground to a reduced thickness.
Conventionally, there have been proposed anchoring and detaching methods for semiconductor wafer and reinforcing member. For example, Japanese Publication for Unexamined Patent Application Nos. 12492/2000 (Tokukai 2000-12492, published on Jan. 14, 2000), and 44144/2001 (Tokukai 2001-44144, published on Feb. 16, 2001) disclose methods in which a UV curable adhesive is used for the bonding, and a reinforcing member is detached by reducing the adhesion force by irradiation of UV light. In another method, a thermoplastic adhesive is used for the bonding, and the reinforcing member is detached by softening the adhesive by applied high-temperature heat after grinding, as disclosed in Japanese Publication for Unexamined Patent Application Nos. 217213/2001 (Tokukai 2001-217213, published on Aug. 10, 2001), 203821/2002 (Tokukai 2002-203821, published on Jul. 19, 2002), and 80938/1994 (Tokukaihei 6-80938, published on Mar. 22, 1994), for example.
In all of these methods, the back surface of the semiconductor wafer is ground while maintaining a strong adhesion force between the semiconductor wafer and reinforcing member. After back grinding, the adhesion force is reduced by irradiation of UV light or application of high-temperature heat, so as to mechanically detach the reinforcing member from the semiconductor wafer while an adhesive layer is still attached on the reinforcing member.
Referring to FIG. 5, the following describes a conventional fabrication method of a thin semiconductor device using a UV curable adhesive.
First, a reinforcing plate 31 is attached via a UV adhesive layer 32, on a front surface 35 of a semiconductor wafer 33 bearing semiconductor devices (not shown) (FIG. 5(a)). Then, while reinforcing the semiconductor wafer 33 with the reinforcing plate 31, a back surface (portion 33b) of the semiconductor wafer 33 is ground to provide a semiconductor wafer 33a of a reduced thickness (FIG. 5(b)). Thereafter, a dicing tape 36 is attached on the back surface (ground surface 41) of the semiconductor wafer 33a (FIG. 5(c)). The dicing tape 36 serves as a support when dividing the semiconductor wafer 33a into individual semiconductor devices.
Next, by irradiation of UV light 46 on the UV adhesive layer 32, the adhesion force of the UV adhesive layer 32 is reduced (FIG. 5(d)). In the next step, a mechanical force is applied on the reinforcing plate 31 to detach the reinforcing plate 31 and the adhesive layer 32 from the semiconductor wafer 33a (FIG. 5(e)). Finally, the semiconductor wafer 33a is diced into individual pieces of semiconductor device 30 (FIG. 5(f)), and the divided pieces of semiconductor device 30 are picked up (FIG. 5(g)).
Note that, the method using a thermoplastic adhesive follows the same steps, except that the method uses a thermoplastic adhesive instead of a UV curable adhesive (adhesive layer 32), and that high-temperature heat is applied instead of UV light.
While the method using a UV curable adhesive can reduce the adhesion force of the adhesive to a certain point by irradiation of UV light, it cannot completely remove the adhesive sticking to the front surface of the semiconductor wafer 33a. That is, a remaining adhesive retains its adhesion force on the front surface of the semiconductor wafer 33a. In this case, when a mechanical force is applied on the reinforcing member 31 to detach the reinforcing member 31 from the semiconductor wafer 33a, the adhesive remaining on the front surface of the semiconductor wafer 33a transmits the mechanical force to the semiconductor wafer 33a, pulling the semiconductor wafer 33a. 
In the step of detaching the reinforcing plate 31 from the semiconductor wafer 33a, the reinforcing plate 31 is attached on the back surface of the semiconductor wafer 33a (see FIG. 5(e)). The dicing tape 36 is not rigid, and the adhesion between the dicing tape 36 and the semiconductor wafer 33a is not strong enough to retain the state (planar shape) of the semiconductor wafer 33a against the mechanical force that transmits to the semiconductor wafer 33a (force pulling the semiconductor wafer 33a). This may cause cracking in the semiconductor wafer 33a when detaching the reinforcing plate 31 from the semiconductor wafer 33a. As one can imagine, the problem of cracking becomes even more serious when the thickness or size of the semiconductor wafer 33a is increased.
Another drawback of the conventional methods is that the reinforcing plate 31 requires a material that transmits UV light 46. That is, the material of the reinforcing plate 31 is limited to UV transmissive materials.
The methods using a thermoplastic adhesive may also cause the problem of cracking in the semiconductor wafer 33a when the reinforcing plate 31 is detached from the semiconductor wafer 33a. 
Further, owning to the fact that the semiconductor wafer 33a and the reinforcing plate 31 have different coefficients of thermal expansion, the semiconductor wafer 33a may be fractured when high-temperature heat (for example, above 100° C.) is applied to reduce the adhesion force of the adhesive.
In order to prevent these problems, Japanese Publication for Unexamined Patent Application No. 222491/1996 (Tokukaihei 8-222491, published on Aug. 30, 1996) discloses a method in which an adhesive layer is dissolved to detach the reinforcing plate.
The method (may be referred to as “conventional method” hereinafter) dissolves the adhesive layer in the manner shown in FIG. 6(a) and FIG. 6(b). First, the semiconductor wafer 33a and adhesive layer 32 are immersed in a solvent 45 that can dissolve the adhesive (FIG. 6(a)). The solvent 45 dissolves the adhesive layer 32, and the reinforcing plate 31 is detached (FIG. 6(b)). In this method, the reinforcing plate 31 is detached from the semiconductor wafer 33a after the adhesive layer 32 is removed. Accordingly, no mechanical force is exerted on the semiconductor wafer 33a when the reinforcing plate 31 is detached.
In the conventional method, however, the reinforcing plate 31 is detached while the adhesive layer 32 and the semiconductor wafer 33a are immersed in the solvent 45. That is, the reinforcing plate 31 needs to be detached without a support, such as a dicing tape, on a back surface 41 of the semiconductor wafer 33a (see FIG. 6(a)). More specifically, the semiconductor wafer 33a, with the reinforcing plate 31 detached and with its thickness reduced, is not supported on a support. This makes it extremely difficult to handle the semiconductor wafer 33a without causing cracking or chipping.
As a countermeasure, a support, such as a dicing tape, may be attached on the semiconductor wafer 33a after the reinforcing plate 31 is detached. However, in this case, the semiconductor wafer 33a may be cracked or bent when the dicing tape is pressed against the back surface of the semiconductor wafer 33a. 
An alternative method is shown in FIG. 7(a) and FIG. 7(b).
As illustrated in FIG. 7(a), after grinding the semiconductor wafer 33, a dicing tape 36 is attached on a back surface 41 of the semiconductor wafer 33a with a reinforcing plate 31 attached on a front surface 35 of the semiconductor wafer 33a. Then, with a support jig 37 covering the dicing tape 36 and the side surface of the semiconductor wafer 33a, a solvent 45 is allowed to gradually permeate through an adhesive layer 32 from the sides to inside the adhesive layer 32, without touching the dicing tape 36. The solvent 45 dissolves the adhesive layer 32, and the reinforcing plate 31 is detached, as shown in FIG. 7(b). In this manner, the method enables the reinforcing plate 31 to be detached from the semiconductor wafer 33a, with the dicing tape 36 attached on the semiconductor wafer 33a. 
The method requires the solvent 45 to gradually permeate through the adhesive layer 32 from the sides to inside the adhesive layer 32. This is problematic in that it takes time to completely dissolve the adhesive layer 32 and thereby enables the reinforcing plate 31 to be detached from the semiconductor wafer 33a. 
Another drawback is that the adhesive layer 32 is not dissolved uniformly when the adhesive layer 32 is gradually dissolved from the periphery. This produces uneven stress over the semiconductor wafer 33a supported by the reinforcing plate 31 and the support jig 37, causing the semiconductor wafer 33a to bend or crack.